Warmer packs obtained by placing a warming composition containing, within a breathable pack, iron powder or other oxidizable metal powder (warming means) as its principal component and generating heat when brought into contact with the oxygen in the air, are widely used as pocket heaters or the like.
Such warmers, while simple to use, are still disadvantageous in that the warming composition is shifted to the bottom of the pack by gravity not only during movement but also in a stationary state when worn on the body, creating discomfort due to a change in shape and adversely affecting performance due to a change in the heat-generating capabilities themselves.
Various attempts have been made to support or sandwich the warming composition with a support material or the like as a means to address these problems.
Examples include (1) a method in which a plurality of pieces of nonwoven fabric made of heat-fused fibers containing vegetable fibers are superposed, and a chemical warming agent is dispersed therein (Japanese Laid-Open Patent Application 2-142561), (2) a method in which a warming agent is dispersed and held on a support sheet having numerous voids (Japanese Laid-Open Patent Application 3-152894), and (3) a method in which a piece of nonwoven fabric having a denser structure is superposed using an adhesive on the lower surface of a nonwoven fabric having numerous voids, a powdered warming composition is dispersed and fixed, another piece of nonwoven fabric is superposed on the uppermost surface thereof, and the assembly is heated and compressed with the aid of a mold press (Japanese Laid-Open Patent Application 8-112303).
However, these methods have the following drawbacks related to the manufacture of a sheet shaped heat generating body and to the resulting sheet shaped heat generating body.
Method 1 is disadvantageous in that combining a plurality of nonwoven fabrics by utilizing the water retentivity of vegetable fibers and the heat fusion properties of synthetic fibers requires complex structuring and processing; fails to adequately bond the nonwoven fabrics when certain types of heat-fusible fibers are used, certain amounts of materials are blended and spun, or the like; and sometimes makes it difficult to form sheet shaped heat generating bodies. Another drawback is that a stiff network structure results and the flexibility is lost when nonwoven fabrics are thoroughly fused under heat.
In the case of method 2, in which a warming composition is dispersed and fixed on a support sheet having numerous voids, it is difficult to securely hold the powder in the voids, and, depending on the porosity of the support sheet, the warming agent sometimes pierces the support sheet and leaks out through the lower surface, or the powder penetrates too deeply into the voids and remains on the upper surface, causing powder leakage.
In the case of method 3, in which a warming composition is supported on a piece of nonwoven fabric having numerous voids, and another piece of nonwoven fabric is superposed on the upper surface thereof, the warming composition does not leak out from the nonwoven fabric, but adhesion between the pieces of nonwoven fabric is inadequate, the layers sometimes separating from each other during fabrication, and the powder leaking out from between the layers of nonwoven fabric in certain applications.
The above situation created a need for the development of a flexible sheet shaped heat generating body and a need for the development of a method for manufacturing such a sheet in which the warming composition can be easily dispersed and held in a uniform configuration, and the warming composition is prevented from leaking.